It would be easy to reject the growing popularity of “no corporate PAC” pledges among Democrats as symbolic at best.

Corporate PAC dollars often add relatively little to candidates’ campaign coffers, and are subject to strict limits and full disclosure in any case. The real corruption threat these days comes not from conventional corporate PACs but from super PACs, which may raise and spend unlimited money if they keep candidates at arm’s length, and from secretive “issue” groups that skirt disclosure rules while spending millions on politics.

Nevertheless, there are potent forces driving the more than 100 Democratic candidates who have promised to reject corporate PAC contributions, including a half-dozen potential 2020 presidential contenders. The “no PAC” pledges may strike some as campaign gimmickry, but they reflect both rising public anger over big money in politics, and the power behind anti-big money messages.

“It’s good policy and good politics,” says Adam Bozzi, communications director for End Citizens United, a PAC that spent $26 million in the last election to back reform-minded Democrats, and that has been urging candidates to take “no corporate PAC” pledges this year. Bozzi acknowledged that “there are certainly bigger” steps that candidates could take to clean up Washington, but that rejecting corporate PAC money signals that a candidate regards reform as a priority—and would follow through on Capitol Hill.

End Citizens United credits Democrat Conor Lamb’s upset victory in Pennsylvania’s special House election last month at least in part to his rejection of corporate PAC dollars, which Lamb stressed as a central theme and featured in both his opening and closing campaign ads. Both Lamb and End Citizens United ran ads attacking Republican Rick Saccone for reportedly abusing a taxpayer-funded expense account as a state legislator, and End Citizens United contrasted Lamb’s “no PAC” pledge with the tens of thousands Saccone had received from corporate PACs.

The group’s polling shows that reducing special interest money in government resonates powerfully with voters in battleground states, and particularly with Independents, who rank it second only to stopping terrorism as a priority.

Several leading progressive groups are pressuring candidates on both sides of the aisle to spell out their positions on political money and election reforms. Every Voice, which opposes big money in politics, is gearing up for a weeklong campaign this summer to call on candidates to “lay out clearly what they are going to do to make politics work for everyday people,” says David Donnelly, the group’s president and CEO.

“They have to make a very clear, tangible commitment to act on this in as big and bold a way as possible,” says Donnelly, who argues that candidates must go beyond corporate PAC pledges to act on specific reforms. A recent scorecard released by Common Cause spotlights the issue by ranking where lawmakers stand on key pieces of democracy legislation, including separate bills to shed light on who’s paying for digital political ads, to protect Special Counsel Robert Mueller from firing, to promote low-dollar donations, to protect voting rights, and to help end partisan gerrymandering.

Of course, Democrats have tried to make hay out of campaign-finance issues before, with limited success. Hillary Clinton laid out a robust platform of political money and ethics reforms, but Donald Trump managed to rail against “the swamp” in Washington in a way that resonated more effectively with voters. Trump has gone on to preside over arguably the most ethically challenged administration in recent memory, but his base voters don’t seem to care.

Whether or not voters buy it, the “no PAC” pledge is emerging as a Democratic litmus test, now that California’s Kamala Harris has become the sixth senator talked about as a 2020 Democratic presidential contender to swear off corporate PAC money. She joins Cory Booker (New Jersey), Maria Cantwell (Washington), Kirsten Gillibrand (New York), Bernie Sanders (Vermont), and Elizabeth Warren (Massachusetts) in making a show of rejecting corporate PAC checks.

A long list of House and Senate Democratic challengers have also released ads rejecting big money. In Ohio’s 15th district, Rick Neal promises viewers that he will take “no handouts from lobbyists,” and “won’t take a dime from Wall Street banks, big oil or the health-care giants.” In another ad, Wyoming Senate hopeful Gary Trauner derides Washington’s culture of “dialing for dollars” from “rich folks and corporate bigwigs,” kicks the phone off his desk with his black cowboy boot, and tosses a wad of $100 bills in the trash, declaring: “We need to get big money out of politics.”

In his challenge to Republican Texas Senator Ted Cruz, House Democrat Beto O’Rourke has both rejected corporate PAC money, and also said “thanks, but no thanks” to super PAC help from environmentalist, Trump-impeachment advocate and Democratic big spender Tom Steyer. O’Rourke, who also belongs to a “NO PAC” caucus and has coauthored legislation that would bar lawmakers and congressional candidates from accepting PAC contributions, outraised Cruz by more than two-to-one in the first quarter of this year.

Some argue that Democrats have done so well with low-dollar fundraising—14 House Democratic hopefuls have outraised the GOP incumbents they are challenging so far—that they won’t need big money to win. Others warn that the GOP outside money machine, which will has already featured millions in spending by such conservative mega-donors as Richard Uihlein and Charles and David Koch, will swamp whatever Democrats raise.

Swearing off PAC money many not cost Democrats a whole lot in actual campaign dollars, and it won’t mean much if they ignore democracy legislation once elected. But Democrats may be discovering, as Trump did, how thirsty voters are for anti-Washington attacks. Says Donnelly: “The issue is resonating with the public in a way that candidates ought to pay attention to.”